The present invention relates to a three-dimensional vision system utilizing coherent optical detection. More particularly, the present invention relates to a real-time, high-resolution, optical scanning vision system capable of providing a 256.times.256.times.256 volume of information about a target at rates approaching 10 frames per second.
Many known vision systems are capable of providing information about a target. For example, television cameras can provide moderate-resolution, two-dimensional images of a target in real time. Likewise, White (structured light) scanners are capable of providing high-resolution images, but not in real time. Basically, all known vision systems must strike a balance between resolution and scan time. Systems providing high-resolution images are not capable of providing real-time scanning, and conversely, real-time systems only provide moderate-to-low resolution images (usually two-dimensional). Thus, applications which require high-resolution, three-dimensional, real-time imaging systems are presently unfulfilled.
Recently, advances in optical technology have enabled the use of coherent (heterodyne) optical detection techniques. Such coherent techniques have provided a 1,000-fold increase in the amount of information able to be detected for each pixel or voxel of the target. The techniques and advantages of optical detection are generally described in the co-pending U.S. application Ser. No. 590,350 entitled "FREQUENCY MODULATED LASER RADAR", the teachings of which are incorporated herein by reference. Additionally, the article entitled "COHERENT OPTICAL DETECTION; A THOUSAND CALLS ON ONE CIRCUIT", by Link and Henry, IEEE SPECTRUM, February 1987, pp. 52-57 describes the present state of optical heterodyne reception. The teachings of this article are also incorporated into this application by reference.
The advantages of coherent optical detection are fundamental. The information carrying capacity of the optical beam reflected from the target is orders of magnitude greater than other available systems. Briefly, the use of optical heterodyne detection allows for optical radiation detection at the quantum noise level. As such, coherent optical systems provide greater range, accuracy, and reliability than many known prior art telemetry and vision systems. For example, coherent optical systems are capable of providing 1,000-times faster scanning for a given precision. This means that the optical beam is not required to dwell upon a specific location on the target for very long in order to obtain sufficient information about the characteristics of that target location. Likewise, coherent optical systems can provide 1,000-times more precision for a given scanning speed. Also, each measurement yields a unique, unambigious reading. Also, rough surfaces may be easily scanned using radar processing techniques. Coherent optical system also can provide a greater scanning range, a greater working depth of field, and may also operate in ambient light conditions.
Briefly, optical heterodyne detection provides a source light beam which is directed to a target and reflected therefrom. The reflected light beam is then mixed with a local oscillator light beam on a photodetector to provide optical interference patterns which may be processed to provide detailed information about the target. Optical heterodyne techniques takes advantage of the source and reflected light beam reciprocity. For example, these light beams are substantially the same wavelength and are directed over the same optical axis. This provides an improved signal-to-noise ratio and sensitivity. The signal-to-noise ratio is sufficiently high so that a small receiving aperture may be used, in contrast to known systems. Since a small receiver aperture can still provide detailed information about the target, the scanning optics of a vision system may be made very small and provide related increases in scanning speed. For example, a coherent optical system using a 1/2" aperture can scan much faster than a 4" aperture used with a direct optical detection system.
Prior art shows that several laser systems have been applied to metrology, and to some extent to gauging. The best known of these is the interferometer which has become a standard for precision measurements. However, the interferometer only measures changes in distance and must be implemented with precisely oriented cooperative reflectors. The proposed invention achieves precise measurement of absolute distances off ordinary and rough surfaces. Other prior art laser applications to gauging achieved distance measurements with incoherent detection and triangulation of a laser source and detection system. The accuracy and versatility of such systems are extremely limited.
Key technologies of AlGaAs laser diodes and fiber optical components are enjoying a burst of development for applications in telecommunications. Because of these efforts, recent improvements in the quality of injection laser diodes provide the coherence length and wave length tuning range needed for a precision, coherent optical scanning system. The small size of the injection laser diode and high-technology integrated optical assemblies make possible the development of a new family of small, low-cost, precise scanning sensors which are orders of magnitude more accurate and more reliable than their more conventional counterparts.
The fundamental concept of coherent optical detection used in the present invention is based on the FM CW radar principle. The FM optical source produces a continuous beam of radiation which is directed at the target. A local oscillator beam is derived from the source light beam and directed to a photodetector. Light reflected from the target is also directed to the photodetector. Since the detector sees energy reflected from the target as well as directly from the source, interference beats are detected as the frequency is swept over the interval .DELTA.f. The rate of these beats is a function of the range as well as the magnitude of the frequency interval. This technique allows a tremendous amount of information concerning the target to be derived from the reflected light beam.
One coherent optical detection system is described in U.S. Pat. No. 4,611,912 to Falk et al. Falk et al '912 describes a method and apparatus for optically measuring a distance to and the velocity of a target. In Falk et al, a laser diode provides a linearly polarized, amplitude modulated (with frequency modulated sub carrier) source light beam. The source light beam is directed to a polarization dependent beam splitter which reflects it toward the target. Between the beam splitter and a target is disposed a quarter wave retardation plate which converts the linearly polarized source light beam to right-hand circularly polarized optical radiation. Between the quarter wave plate and the target, a local oscillator reflector plate reflects approximately 1% of the source light beam back toward the beam splitter, while allowing approximately 99% of the source light beam to pass toward the target. Light reflected from the target and the local oscillator beam are thereby converted from right-hand circularly polarized optical radiation to left-hand circularly polarized optical radiation. These beams then pass back through the quarter wave plate and are thereby converted to linearly polarized light beams. These linearly polarized light beams pass through the polarizing beam splitter and are concentrated on a PIN diode by a collecting optical lens. Thus, the local oscillator and the return beam are both linearly polarized in the same direction and are directed along the same optical axis. Thus, the PIN diode detects an optically mixed signal containing the local oscillator beam and the light beam reflected from the target.
However, an extreme disadvantage of the Falk et al '912 system is that very close alignment is required between the optical components. Thus, the laser diode, the beam splitter, the quarter wave plate, the local oscillator reflecting plate, and the PIN diode must be carefully adjusted before usable signals may be obtained. In addition, such close adjustment allows for rapid system degradation with temperature changes and mechanical shocks. Additionally, the Falk et al '912 system only provides apparatus for measuring distances and velocity. Such a system would be difficult to adapt to a scanning vision system since scanning components must be added which would further exacerbate the optical alignment sensitivity problems noted above.
U.S. Pat. No. 4,594,000 to Falk et al also discloses a system for optically measuring the distance to and velocity of a target. This system is somewhat like Falk et al '912, but incorporates a reference arm to provide more precise measurements of distance and velocity. Specifically, the FM source light beam is also provided to a reference arm which also includes a polarization-dependent beam splitter, a quarter wave plate, and a local oscillator reflecting mirror. The local oscillator reflecting mirror again reflects a local oscillator beam back through the beam splitter to a PIN diode. The reference source light beam is allowed to propagate through the local oscillator reflecting mirror into a fiber optic coil of known length having a reflecting element at the end thereof. The reference source light beam is reflected back through the fiber optic coil and mixes with the local oscillator beam at the PIN diode. Means are then provided to determine a representative value of the frequency of the output signal from both the target and reference arms. A processor is then utilized to calculate the distance and velocity of the target from the representative frequency values derived from both the source and the reference optical heterodyne systems. While this system is capable of providing great precision in velocity and distance measurements, it also suffers the optical alignment sensitivity problems of Falk et al '912.
Therefore, for a practical, three-dimensional, scanning vision system, there is a requirement for an optical detection system whose optical alignment sensitivity is extremely low.